Broad Objectives. We are attempting to develop an understanding of the interaction between Bacteroides fragilis and its host. What factors associated with this opportunistic pathogen allow it to establish infection more frequently than other aerotolerant anaerobes? Our approach is to explore the inflammatory response of the animal evoked by the invading microorganism. Sterile Bacteroides culture filtrates and isolated crude outer membrane have a direct chemotactic effect upon PMN neutrophils; however, in the presence of serum complement, PMN locomotion is stopped. Exocytosis of PMN granules is not negatively effected by this combination of reactants. Since we know that the lipopolysaccharide is not causing this effect, we intend to separate the outer membrane and determine which component (capsular polysaccharide or protein) is causing this negative effect. We will determine what effect the outer membrane is having on the C5a complement fraction. Bacterial age versus competence phenomenon will be explored via chemotaxis and abscess initiation. There may be a physical difference between the young and old cells (more capsule produced as they get older and they respond less to serum-killing with complement). The effect of depleting mice of complement will be compared for ability to produce abscesses with young and old cells.